1. Field of the Invention
The present invention relates generally to heat sinks used to dissipate heat from semiconductor devices during normal operation thereof. Particularly, the present invention pertains to the use of stereolithographic techniques to fabricate heat sinks for use on semiconductor devices, to heat sinks so fabricated, and to semiconductor devices including stereolithographically fabricated heat sinks.
2. State of the Art
During normal use, semiconductor devices generate heat. Adequate dissipation of the heat generated during normal use of a semiconductor device is necessary for the safe and reliable operation of an electronic appliance that includes the semiconductor device. If the semiconductor device reaches an excessively high temperature, the integrated circuits of the semiconductor device can fail or a circuit board fire can result, damaging the electronic system of which the semiconductor device is a part.
While some semiconductor devices are able to dissipate sufficient amounts of heat without an additional heat sink or heat spreader, state of the art semiconductor devices with increased speed, circuit complexity, and circuit density often require added heat sinks.
In particular, as semiconductor devices have become more dense in terms of electrical power consumption per unit volume, heat generation has greatly increased, requiring package construction which dissipates the generated heat much more rapidly. As the state of the art progresses, the ability to adequately dissipate heat is often a severe constraint on the size, speed, and power consumption of an integrated circuit design.
In this application, a heat sink will be distinguished from a xe2x80x9cheat spreader,xe2x80x9d the former pertaining to a structure with a heat transfer portion or element positioned proximate to a semiconductor device and a heat dissipation portion or element relatively more remote from the semiconductor device, the latter pertaining to a member which channels heat from a semiconductor die to leads which exit the die package. However, a heat sink and a heat spreader may together be used to cool a device.
Typically, heat sinks are fabricated from materials with good thermal conductivity, such as metals (e.g., aluminum, copper alloys, etc.), ceramic materials, and glass. The heat transfer portion of a heat sink is configured to absorb heat from the semiconductor device proximate thereto and, therefore, generally contours to at least a portion of a surface of the semiconductor device. The heat dissipation portion of a heat sink may include a series of small protrusions, which are typically referred to as xe2x80x9cfins,xe2x80x9d that receive heat from the heat transfer portion of the heat sink and are configured to dissipate the heat away from the semiconductor device as air flows between the fins. The shapes, sizes, arrangement, spacing, and numbers of fins on a heat sink are configured so as to optimize the heat dissipation capabilities of the heat sink with respect to the particular heat dissipation needs of a specific type of semiconductor device.
Heat sinks are typically manufactured separately from the semiconductor devices to which they are subsequently secured.
Conventionally, metal heat sinks have been manufactured by extrusion or casting processes. When extruded, molten metal is forced through an extrusion die to produce an elongated extrusion of a cross-section taken transverse to the length thereof of a desired heat sink configuration. The elongate extrusion is then sectioned transverse to the length thereof to provide the heat sinks. Cast heat sinks are manufactured by disposing a molten quantity of heat conductive material into a refractory mold.
Heat sinks can also be machined from blocks of material. As conventional heat sinks have spaced apart fins, however, machining processes waste a considerable amount of material. In addition, due to the small size and high complexity of conventional heat sinks, the use of machining processes can be very time consuming and expensive. For these reasons, the use of machining processes to manufacture heat sinks is somewhat undesirable.
The use of extrusion, casting, and machining processes to manufacture heat sinks are also somewhat undesirable since each of the processes limit the possible configurations of the manufactured heat sinks. For example, when extrusion is used, the transverse cross-section taken along the entire length of each heat sink has the same two-dimensional shape, being that imparted by the two-dimensional configuration of the extrusion die. When heat sinks are cast, the configurations thereof are determined by the casting molds. Typically, molds have two parts, and may include additional inserts to facilitate the formation of more complex features. State-of-the-art machining processes are limited to, at most, seven axes. Typically, however, less complex three-axis or five-axis machines are used. Nonetheless, certain types of features, such as internally confined cavities and non-linear channels cannot be formed easily when casting or state-of-the-art machining equipment is used.
An alternative method for manufacturing heat sinks is disclosed in U.S. Pat. No. 5,814,536, issued to Rostoker et al. on Sep. 29, 1998 (hereinafter xe2x80x9cthe ""536 Patentxe2x80x9d). The ""536 Patent discloses the use of powder metallurgy techniques to form a heat sink. Thus, the heat sink is formed from a mixture of powdered metal (e.g., copper, aluminum, tungsten, titanium, and alloys thereof) and a suitable binder. The mixture is placed into a mold, where the metal particles are bonded to adjacent particles, or sintered together, under appropriate pressure and at an appropriate temperature. The binder, if any, is removed (i.e., burned off) during the sintering process. The sintered heat sink can then be machined to provide features that may not be readily obtained or possible to obtain by the sintering process alone. Since the sintering process of the ""536 Patent employs a mold, it is somewhat undesirable due to the previously mentioned conformational limitations that are present when a mold is used.
As noted above, a prefabricated heat sink is conventionally assembled with a semiconductor device. The assembly can then be packaged by known techniques, such as by transfer molding of a particle-filled polymer, as known in the art. If such an assembly is packaged, however, the packaging mold must usually be configured so as to receive at least a portion of the heat sink to permit its projection beyond the polymer packaging. The manufacture of molds configured to receive heat sinks is somewhat undesirable due to the complexity of the mold designs and the high costs of machining such molds.
The art does not teach a method of fabricating heat sinks on semiconductor devices or of fabricating heat sinks by stereolithography, or layered manufacturing, processes.
In the past decade, a manufacturing technique termed xe2x80x9cstereolithography,xe2x80x9d also known as xe2x80x9clayered manufacturing,xe2x80x9d has evolved to a degree where it is employed in many industries.
Essentially, stereolithography as conventionally practiced involves utilizing a computer to generate a three-dimensional (3-D) mathematical simulation or model of an object to be fabricated, such generation usually effected with 3-D computer-aided design (CAD) software. The model or simulation is mathematically separated or xe2x80x9cslicedxe2x80x9d into a large number of relatively thin, parallel, usually vertically superimposed layers, each layer having defined boundaries and other features associated with the model (and thus the actual object to be fabricated) at the level of that layer within the exterior boundaries of the object. A complete assembly or stack of all of the layers defines the entire object, and surface resolution of the object is, in part, dependent upon the thickness of the layers.
The mathematical simulation or model is then employed to generate an actual object by building the object, layer by superimposed layer. A wide variety of approaches to stereolithography by different companies has resulted in techniques for fabrication of objects from both metallic and non-metallic materials. Regardless of the material employed to fabricate an object, stereolithographic techniques usually involve disposition of a layer of unconsolidated or unfixed material corresponding to each layer within the object boundaries, followed by selective consolidation or fixation of the material to at least a partially consolidated, fixed, or semisolid state in those areas of a given layer corresponding to portions of the object, the consolidated or fixed material also at that time being substantially concurrently bonded to a lower layer of the object to be fabricated. The unconsolidated material employed to build an object may be supplied in particulate or liquid form, and the material itself may be consolidated or fixed, or a separate binder material may be employed to bond material particles to one another and to those of a previously-formed layer. In some instances, thin sheets of material may be superimposed to build an object, each sheet being fixed to a next lower sheet and unwanted portions of each sheet removed, a stack of such sheets defining the completed object. When particulate materials are employed, resolution of object surfaces is highly dependent upon particle size, whereas when a liquid is employed, surface resolution is highly dependent upon the minimum surface area of the liquid which can be fixed and the minimum thickness of a layer that can be generated. Of course, in either case, resolution and accuracy of object reproduction from the CAD file is also dependent upon the ability of the apparatus used to fix the material to precisely track the mathematical instructions indicating solid areas and boundaries for each layer of material. Toward that end, and depending upon the layer being fixed, various fixation approaches have been employed, including particle bombardment (electron beams), disposing a binder or other fixative (such as by ink-jet printing techniques), or irradiation using heat or specific wavelength ranges.
An early application of stereolithography was to enable rapid fabrication of molds and prototypes of objects from CAD files. Thus, either male or female forms on which mold material might be disposed might be rapidly generated. Prototypes of objects might be built to verify the accuracy of the CAD file defining the object and to detect any design deficiencies and possible fabrication problems before a design was committed to large-scale production.
In more recent years, stereolithography has been employed to develop and refine object designs in relatively inexpensive materials, and has also been used to fabricate small quantities of objects where the cost of conventional fabrication techniques is prohibitive for same, such as in the case of plastic objects conventionally formed by injection molding. It is also known to employ stereolithography in the custom fabrication of products generally built in small quantities or where a product design is rendered only once. Finally, it has been appreciated in some industries that stereolithography provides a capability to fabricate products, such as those including closed interior chambers or convoluted passageways, which cannot be fabricated satisfactorily using conventional manufacturing techniques. It has also been recognized in some industries that a stereolithographic object or component may be formed or built around another, pre-existing object or component to create a larger product.
However, to the inventor""s knowledge, stereolithography has yet to be applied to mass production of articles in volumes of thousands or millions, or employed to produce, augment or enhance products including other, pre-existing components in large quantities, where minute component sizes are involved, and where extremely high resolution and a high degree of reproducibility of results is required. In particular, the inventor is not aware of the use of stereolithography to fabricate heat sinks for use with semiconductor devices. Furthermore, conventional stereolithography apparatus and methods fail to address the difficulties of precisely locating and orienting a number of pre-existing components for stereolithographic application of material thereto without the use of mechanical alignment techniques or to otherwise assuring precise, repeatable placement of components.
According to one aspect, the present invention includes a method for fabricating heat sinks for use with semiconductor devices. In a preferred embodiment of the method, a computer-controlled, 3-D CAD initiated process known as xe2x80x9cstereolithographyxe2x80x9d or xe2x80x9clayered manufacturingxe2x80x9d is used to fabricate the heat sinks. When stereolithographic processes are employed, a heat sink is formed as a series of superimposed, contiguous, mutually adhered layers of material.
As it is important that heat sinks absorb heat from a proximate semiconductor device and dissipate the heat, the heat sinks of the present invention are preferably manufactured from materials that are good heat conductors. Accordingly, the stereolithography processes that are preferred for fabricating the heat sinks of the present invention are capable of fabricating structures from materials with good thermal conductivity.
In one such stereolithography process, known as xe2x80x9cselective laser sinteringxe2x80x9d or xe2x80x9cSLS,xe2x80x9d structures are fabricated from layers of powdered or particulate material. The particles in selected regions of each of the layers can be bonded together by use of a laser under the control of a computer. The laser either heats the material particles and sinters adjacent particles together, heats a binder material mixed in with the particles to bond the particles, or heats a binder material with which the material particles are coated to secure adjacent particles in the selected regions of a layer to one another.
Another exemplary stereolithography process that may be used to fabricate heat sinks incorporating teachings of the present invention is referred to as xe2x80x9claminated object manufacturingxe2x80x9d or xe2x80x9cLOM.xe2x80x9d Laminated object manufacturing involves the use of a laser or other cutting device to define the peripheries of a layer of an object from a sheet of material. Adjacent layers of the object are secured to one another to form the object.
The stereolithographic heat sink fabrication method of the present invention preferably includes the use of a machine vision system to locate the semiconductor devices or substrates upon which heat sinks are to be fabricated, as well as the features or other components on or associated with the semiconductor devices or substrates (e.g., bond wires, leads, etc.). The use of a machine vision system directs the alignment of a stereolithography system with each semiconductor device or substrate for material disposition purposes. Accordingly, the semiconductor devices or substrates need not be precisely mechanically aligned with any component of the stereolithography system to practice the stereolithographic embodiment of the method of the present invention.
In a preferred embodiment, the heat sink to be fabricated upon a semiconductor device component in accordance with the invention is fabricated using precisely focused electromagnetic radiation in the form of a laser under control of a computer and responsive to input from a machine vision system, such as a pattern recognition system, to define each layer of the object to be formed from a layer of material disposed on the semiconductor device or substrate.
According to another aspect, the present invention includes stereolithographically fabricated heat sinks, as well as semiconductor devices that include stereolithographically fabricated heat sinks. As stereolithographic processes are used to fabricate these heat sinks, the heat sinks may be formed with features that cannot be defined by use of conventional extrusion, sintering, or machining processes.